The fluid catalytic cracking (FCC) process has become well-established in the petroleum refining industry for converting higher boiling petroleum fractions into lower boiling products, especially gasoline.
In the fluid catalytic process, a finely divided solid cracking catalyst is used to promote the cracking reactions which take place in the feed. The catalyst is used in a very finely divided form, typically with a particle size range of 20-300 microns, with an average of about 60-75 microns, in which it can be handled like a fluid (hence the designation FCC) and in this form it is circulated in a closed cycle between a cracking zone and a separate regeneration zone. In the cracking zone, hot catalyst is brought into contact with the feed so as to effect the desired cracking reactions after which the catalyst is separated from the cracking products which are removed from the cracking reactor to the associated fractionation equipment for separation and further processing. During the cracking reaction, coke is deposited on the catalyst. This deposit of coke masks the active sites and temporarily deactivates the catalyst. Such temporarily deactivated catalyst is commonly called spent catalyst. The catalyst must then be regenerated before it can be reused.
Cracking is an endothermic reaction. Heat for the cracking reaction is usually supplied by the regeneration step. Spent catalyst is oxidatively regenerated to remove the coke. The regeneration takes place in a separate regenerator vessel. Catalyst is maintained in one or more fluidized beds in a regenerator vessel and an oxygen-containing gas, usually air, flows through a distribution grid which mixes air with the spent, coked catalyst. During the regeneration step, the coke is burned and the heat of combustion heats the catalyst. The hot, regenerated catalyst is recycled to the cracking zone to crack fresh, and perhaps recycled feed. Thus, the catalyst circulates continuously between the cracking reactor and the regenerator. The heat for the endothermic cracking reaction is supplied by the exothermic regeneration of the catalyst. Most FCC units, and the moving bed analogue, the Thermofor Catalytic Cracking process, operate in a heat balanced mode, with essentially all of the heat needed for the cracking reaction supplied by the heat released in the catalyst regenerator.
A further description of the catalytic cracking process and the role of regeneration may be found in the monograph, "Fluid Catalytic Cracking With Zeolite Catalysts", Venuto and Habib, Marcel Dekker, N.Y., 1978. Reference is particularly made to pages 16-18, describing the operation of the regenerator and the flue gas circuit.
The FCC process is a mature, but still rapidly changing workhorse in modern refineries. Two areas of concern are the wish to use heavier, cheaper feeds, and the desire to improve the value of some light products. Each area will be briefly reviewed.